Process of casting refractory materials



United States Patent C) M 3,351,688 PRGCESS F CAS'ETKNG REFRACTURYMATEREALS William D. Kingery, Lexington, and Arthur Waugh,

Brookline, Mass, assignors to Lexington Laboratories, inc, Cambridge,Mass, a corporation of Massachusetts No Drawing. Filed Sept. 18, 1964,Ser. No. 397,640

4 Claims. (Cl. 264--63) This invention relates to a method of formingarticles composed of sintered refractory materials and more particularlypertains to a process for casting refractory materials in a mannerpermitting the casting to be of complex shape and permitting closecontrol of the dimensions of the casting without necessitating machiningof the article while it is in the green state.

Many methods of casting and sintering refractory materials have beenemployed in the past to produce ceramic and metallic articles.Conventional casting methods have proved to be adequate for producingthe bulk of commercial ceramic ware as that ware is usually simple inshape, is not required to have highly precise dimensions, and is made ofmaterials that are relatively easy to work. In the past, casting ofrefractory materials was done by causing a fine powder of the refractorymaterial to be consolidated into a coherent mass of putty-likeconsistency, pressing the material into a form or die to cause thematerial to be molded into an approximation of the desired finishedshape, causing the piece to harden to a state where it could be groundor otherwise machined to its final configuration, and heating the greenpiece at a high temperature to cause the grains of powder to fusetogether. Machining of the green piece is required to obtain complexshapes as conventional methods do not permit intricate configurations tobe obtained directly by casting. Further, due to wear of the die or formand because of uncertain shrinkage of the green piece during theprocess, castings of highly precise dimensions cannot be produced byconventional methods. To obtain precise dimensions, the casting producedby conventional methods is usually deliberately made oversize and isthen ground to the desired size. Grinding of the casting is expensiveand difiicult as sintered refractory materials tend to be extremely hardand brittle. In attempting, by conventional methods, to produce castingsof shapes having sharp corners or having portions of smallcross-section, it was found that strains are introduced in the greenpiece during drying which cause cracks to appear when the casting isfired into its final form. To produce articles having irregularcontours, undercuts, internal holes, sharp corners, or severe bends,machining operations upon the green piece had to be performed.

The present invention resides in a process for forming objects ofrefractory materials which, without machining of the green or finishedpiece, permits intricately shaped articles of highly precise dimensionsto be produced. In the novel process, the refractory material, in theform of a fine powder, is mixed with a binder and a defiocculating agentto form a slurry. The defiocculant may be oleic acid and the binder is amaterial, such as paratfin, having a volume just sufiicient to fill theinterstices of the looselypacked powdered refractory material when thebinder is solidified and the grains of the refractory material are incontact with one another. In order to maintain the mixture as a slurry,it is kept at a temperature where the binder is liquid. In the slurrythe powdered refractory material is dispersed by the deilocculant sothat the grains are distributed homogeneously throughout the binder. Theslurry is cast into a mold of the desired shape and the binder ispermitted to solidify so that a green piece is formed having a uniformdensity. The slurry, being a 335L688 Patented Nov. 7, 1967 liquidsuspension, conforms closely to the configuration of the mold so thatthe green piece can have intricate shapes, corners, and sharp bends. Thegreen piece, after removal from the mold, is packed in an inertrefractory powder, such as alumina, and is then heated slowly to atemperature at which the binder vaporizes and is driven off through avent. A low rate of increase of temperature is employed to prevent thebinder from vaporizing so rapidly as to weaken or rupture the casting.After a length of time sufficient to drive ofli all the binder, thetemperature is raised to a level where partial sintering occurs. Partialsintering alfords sufiicient strength in the casting to permit it to beremoved from the alumina packing and the alumina to be dusted from thepresintered casting. Subsequently, the casting is heated to sinteringtemperature and there maintained for a period of time determined by thedesired density of the finished article.

Castings having intricate shapes may be made by the process heredescribed and irregular contours, undercuts, internal bores, and threadscan be produced. Virtually any shape and size can be reproduced,depending principally upon the skill of the die maker. Further, becausethe powder is homogeneously dispersed in the hinder the resultant greencasting is of uniform density, and its shrinkage from the green state tothe size of the finished casting does not result in stresses which crackthe casting and permits the final size of the finished product to beknown with a large degree of certainty. The shrinkage of the sinteredcasting is, in many instances, less than 1% of its volume in the greenstate.

This process may also be adapted to other forming methods. For example,hollow bodies such as beakers or thermocouple housing may be produced byusing a form whose exterior dimensions conform to desired interiordimensions of the final product, coating the form with a parting agentsuch as silicone grease and slipping the form into the molten slurry. Tobuild up a significant thickness of material 011 the surface of theform, the form should be cooled below the melting point of the slurry.After a sufficient thickness of the solidified slurry is built up on theexterior of the form, it is removed and handled as previously described.

The amount of parafiin or other binder used in the slurry is such as tofill the interstices of the powdered refractory material and the volumeratio of powder to binder is of major importance. The ideal ratio is onewhere when the particles of powder are in contact with each other andthe binder is just sufiicient to occupy the interstitial spaces. If theratio is too low, then movement of the powder grains will occur when thebinder is driven off, causing the piece to distort and lose itsdimensions. If the ratio of powder to binder is too high, not enoughbinder will be present to completely fill the interstices so that whenthe slurry within the die solidifies, the green piece is not of uniformdensity. Furthermore, the slurry tends, when the ratio of powder is toohigh, to be viscous and, therefore, may not conform to the shape of thedie.

When the binder is driven off, the green piece has no strength andcannot be handled. It is necessary, therefore, prior to driving oflf thebinder, to put the green piece in an inert packing. A packing powder isused which does not react with the casting and which is composed ofgrains of such size as to give a good surface finish to the casting.Further, the packing powder must have a sintering temperature above thatof the material of the casting because the casting is partially sinteredimmediately after all the binder has been driven off.

Specific examples of the process as applied to various materials are setforth below. It should be understood that the examples which are givenillustrate the capabilities of the process are are not intended aslimitations upon the scope of the invention.

Example I A granular powder of tungsten is uniformly dispersed in abinder treated with a deflocculant. The binder is a material that is asolid at room temperature and is characterized by a low melting point, alow viscosity when molten, and a vaporization point below thepresintering temperature of the powder used. The binder is preferablychosen from the waxes or paraffins and wherever parafiin is employed inthe specification, a binder having the foregoing characteristics isintended. The defiocoulant may be any dispersing agent that is misciblewith the hinder and which acts to provide complete and uniform wettingof the powder \grains.

One thousand eight hundred and eighty grams (1880) grams of tungstenpowder, milled to a fineness permitting it to pass through a screen ofthree hundred and twentyfive (325) mesh, is mixed in a molten solutionof one hundred (100) grams of a paraffin binder and fourteen and twotenths (14.2) grams of oleic acid to form a .slurry. Where a greater orlesser amount of the slurry is required, the quantities are determinedby measuring the volume of the powdered tungsten, ascertaining theamount of paraffin which in its solid state is equal to about 45% of thepowders volume, and using about 1% by weight of oleic acid. It has beenfound that if the volume of binder is less than this, the viscosity ofthe slurry at the melting point of the parafiin is too high for easyflow and if the volume of binder is significantly higher than thisproportion, large shrinkage occurs upon driving off the binder. Thevolume of binder in its solid form should be equal to the volume of theinterstices of the granular tungsten. When in the molten state, thevolume of binder should be sufficient to permit dispersing the grainssufiiciently so that the viscosity of the mixture is such that theslurry can be poured freely. The temperature of the slurry is keptbetween 250 and 300 C. and the slurry is constantly agitated so as tomaintain turbulence for a time sufficient for the slurry to outgas. Aperiod of thirty minutes was found to be adequate for outgassing theslurry. The temperature should not be allowed to exceed the 300 C. atwhich vaporization of the paraffin begins. Following outgassing of theslurry, it may other parting agent. The material in the mold ispermitted to cool and solidify. The solid casting is removed fro-m themold and packed in alumina powder that has been passed through a screenof at least 100 mesh. The package is heated slowly to a temperature of400 C. to drive off the binder and the deilocculant. It should be notedthat binder and the defiocculant vaporize without leaving anyappreciable residue. The package is vented to permit the vapors toescape and the temperature is raised slowly so that rapid vaporizationdoes not occur. A rate of temperature increase of 1.5 C. per minute issatisfactory. After all the binder and deflocculant are driven off, thetemperature is increased to about 1700" C. and there maintained for aperiod of about three hours. The presintering, at about 1700" C., isperformed in order to impart enough rigidity to the casting to permit itto be handled.

Subsequent to presintering, the casting is removed from the packing and,after being dusted to remove any adherent alumina particles, is fired atsintering temperature for a time determined by the desired density ofthe finished "casting. In this example, a density of 72% is obtained byfiring the casting at 2200 C. for one hour.

Example II A ceramic casting was made by the procedure describedhereinafter. Granules of zirconium dioxide (ZrO which had passed througha screen of 325 mesh were employed. To a melt consisting of grams ofparaflin and 2.8 grams of oleic acid was added 448 grams of screenedzirconium dioxide. As described in the previous example, the melt wasprepared by heating the paraffin until it was liquid and the oleic acidwas thoroughly mixed with it. The powdered zirconium dioxide was putinto the melt and the slurry was heated for a time sufficient to permitthe temperature to stabilize somewhere between 250 and 300 C. and forall the air that was entrained in the mixture to escape. During thistime, the slurry was agitated by stirring to assure that the powder wasdispersed in the melt. The slurry was then poured into a mold that hadbeen treated with silicone grease to insure that the casting could beeasily parted from the mold. The slurry was permitted to cool andsolidify. Following the solidification of the mixture, the casting wasremoved from the mold and packed in a refractory powder, again alumina,and raised very slowly from 25 C. to 1100 C. over a 24-hour period.Following the cooling of the casting, which has now been dewaxed, thecasting was removed from the alumina and reheated from 25 C. to 1500 C.and maintained at that temperature for 18 hours whereupon a 72% densityand 1.7% shrinkage was achieved.

Other materials have also been used for producing castings: for example,magnorite, forsterite, mullite and fused silica are just a few of themany types of materials that may be used following this disclosedprocess. Densities and shrinkage of the final product have varied, thefollowing of which are typical examples:

Magnesium oxide devices have been built in which 68% of the theoreticaldensity has been achieved with an .8% firing shrinkage, while mixturesof aluminum trioxide and silicon dioxide have been fired together withtheoretical densities of and a firing shrinkage of only .7%. I

It should be understood although both examples described a granularpowder sufficient to pass through a three hundred and twenty-five (325)mesh, the coarser or finer powders may be used.

Although this invention has been described in connection with specificexamples, it can be readily recognized that the invention is capable ofa wide variety of modifications and variations and should be limited inscope only by the appended claims.

What is claimed is:

1. A process for forming an object of precise predeterminedconfiguration comprising the steps of: mixing a refractory granularmaterial capable of being sintered with a meltable binder in sufficientquantities so that the volume of said binder when in a solid state isequal to the volume of the interstices of said material forming saidmixture in a molten state into said configuration, allowing said mixtureto solidify thereby forming a solid object having said predeterminedconfiguration, packing said object into a non-reactive refractorypowder, heating said packed object to a temperature sufiiciently highenough to drive off said binder while leaving said granular material,raising the temperature of said object to a level for presintering tounify said material and thereafter removing said object from the powderand heating said object to a temperature for sufficient time for saidobject to reach a desired density.

2. A process of forming an object of predetermined configurationcomprising the steps of: mixing a paraffin and a suspending agent with arefractory granular material capable of being sintered in suflicientamounts so that the volume of the paraffin and the suspending agent areequal to the volume of interstices of said granular material when themixture is at room temperature heating the mixture to a viscosity levelat which the mixture will freely pour, pouring said mixture into a moldhaving the predetermined configuration, allowing the mixture to solidifyin said mold, removing the solid mixture from the mold, packing thesolid in a nonreactive material, heating the casting to a temperaturesufiicient to volatilize said paraffin and said suspending agent but notsuflicient to affect the granular material, maintaining said temperatureuntil the paraflin and said suspending agent is driven off, increasingthe temperature to a presintering level, allowing said solid to cool,and thereafter removing the object from the non-active material, heatingthe casting to a tem perature for a sufiicient time to sinter saidobject.

3. A process of making ceramic bodies of a predetermined configurationconsisting of the steps of: preparing a solution of parafiin and oleicacid, heating said solution to a temperature of greater than 250 C. butless than 300 C. to place said solution in a molten state, forming amixture by adding a ceramic material in granular form to said moltensolution, agitating said mixture to create turbulence therein,maintaining said turbulence in said mixture for a period of timesufiicient to homogenize and stabilize the temperature of said mixtureand to free entrapped gas from said mixture, coating a mold of saidconfiguration with a parting agent, pouring said mix ture into said moldwhile said mixture is in a molten state, allowing said mixture to assumethe shape of said mold and to solidify therein with said solidifiedsolution just filling the interstices of said ceramic granules, removingsaid solid mixture from said mold, placing the molded object in apacking of -a nonreactive, refractory, granular material to support saidsolid mixture, heating to a presintering temperature sufficient to dewaxsaid object and to impart structural rigidity to the object, causing thetemperaturre of the object to cool to 25 C., removing said solid fromsaid packing, and heating said solid at sintering temperature to achievea desired density.

4. A material suitable for molding with a shrinkage rate of less than10% comprising:

a refractory granular material capable of being sintered, and a binder,said granular material being dispersed throughout said hinder, therelative amounts of said binder and said granular material being suchthat. the volume of said binder in a solid state is equal to the volumeof the interstices of said granular material at room temperature andwherein said binder has as its major constituent, paraflin.

References Cited UNITED STATES PATENTS 2,122,960 7/1938 Schwartzwalder2646! 2,422,809 6/1947 Stupakoif et a1 26463 X 3,051,566 8/1962 Schwartz26463 XR 3,234,308 2/1966 Herrmann 26463 3,252,809 5/1966 Somers 26463 XFOREIGN PATENTS 137,807 1961 Russia. 150,047 1962 Russia.

ROY B. MOFFITT, Primary Examiner,

ALEXANDER H. BRODMERKEL, ROBERT F.

WHITE, Examiners J. A. FINLAYSON, Assistant Examiner.

1. A PROCESS FOR FORMING AN OBJECT OF PRECISE PREDETERMINED CONFIGURATION COMPRISING THE STEPS OF: MIXING A REFRACTORY GRANULAR MATERIAL CAPABLE OF BEING SINTERED WITH A MELTABLE BINDER IN SUFFICIENT QUANTITIES SO THAT THE VOLUME OF SAID BINDER WHEN IN A SOLID STATE IS EQUAL TO THE VOLUME OF THE INTERSTICES OF SAID MATERIAL FORMING SAID MIXTURE IN A MOLTEN STATE INTO SAID CONFIGURATION, ALLOWING SAID MIXTURE TO SOLIDIFY THEREBY FORMING A SOLID OBJECT HAVING SAID PREDETERMINED CONFIGURATION, PACKING SAID OBJECT INTO A NON-REACTIVE REFRACTORY POWDER, HEATING SAID PACKED OBJECT TO A TEMPERATURE SUFFICIENTLY HIGH ENOUGH TO DRIVE OFF SAID BINDER WHILE LEAVING SAID GRANULAR MATERIAL, RAISING THE TEMPERATURE OF SAID OBJECT TO A LEVEL FOR PRESINTERING TO UNIFY SAID MATERIAL AND THEREAFTER REMOVING SAID OBJECT FROM THE POWDER AND HEATING SAID OBJECT TO A TEMPERATURE FOR SUFFICIENT TIME FOR SAID OBJECT TO RACH A DESIRED DENSITY. 